1. Field of the Invention
The present invention relates to a vibration-type actuator for use in camera lenses and office automation (OA) equipment in which an electro-mechanical energy conversion element produces an elliptic motion on a surface of a vibration member to relatively move the vibration member and a driven member. The present invention also relates to an imaging apparatus.
2. Description of the Related Art
In recent years, a vibration-type actuator (vibration wave driving apparatus) has been used in various fields. In the vibration-type actuator, a distortion generating element that generates a mechanical distortion in response to the action of an electric or magnetic field vibrates a vibration member, and the vibration of the vibration member is converted into a continuous or intermittent mechanical motion and then output.
Among piezoelectric actuators using a piezoelectric element, an actuator called an ultrasonic motor can constitute a continuous rotation driving source.
Hence, the ultrasonic motor has been used as a driving source as a substitute for a conventional rotary electromagnetic driving motor in an optical apparatus such as a camera. Driving control techniques for the ultrasonic motor have practically been established.
Although techniques relating to ultrasonic motors have practically been established, further improvement is necessary in motor performance stabilization techniques.
A vibration-type actuator (ultrasonic motor) that produces bending-mode vibrations of the same shape in a plurality of different planes is discussed in publications such as Japanese Patent Application Laid-Open No. 4-91671. Drive principles are discussed in detail in the publications. Vibration members discussed in the publications include an electro-mechanical energy conversion element and elastic members. In a vibration member, the electro-mechanical energy conversion element is sandwiched and fixed by the elastic members in both sides. The vibration-type actuator that produces bending-mode vibrations of the same shape in a plurality of different planes enables driving by generating an elliptic motion of surface particles of the vibration member, bringing a driven member into pressure contact with the vibration member, and continuously driving the driven member.
In a piezoelectric element, a pattern electrode is formed, and substantially sinusoidal alternating voltages with time phases that differ by 90 degrees are sequentially applied to each electrode.
When alternating voltages having frequencies near the natural vibration frequencies of the vibration modes to be produced are applied, the vibration member is resonated by the bending moment applied to the vibration member by expansion and contraction of the piezoelectric element.
The vibration modes produced with respect to the alternating voltages that differ by 90 degrees have the same shape and different phases, and are combined to generate an elliptic motion of the surface particles of the vibration member. The vibration modes of the vibration-type actuator that produces bending-mode vibrations of the same shape in a plurality of different planes have the same deformation distribution. Hence, the vibration-type actuator has a characteristic that the resonance frequencies are less likely to be changed by the vibration direction and, thus, substantially no adjustment is required to match the resonance frequencies of two modes.
For the vibration-type actuator to perform highly-accurate driving, friction contact surfaces of the driven member and the vibration member are desirably vertical to the direction of a shaft and have no inclination with respect to the shaft. Hence, both end surfaces of a coil spring of a pressurization spring are polished to reduce inclination. For high responsiveness, an external output is output by a gear without a spring and a vibration-proof rubber. Compared with a plate spring and a disc spring, the coil spring enables easy adjustment of pressing force because a large stroke can be set and the spring constant can be decreased. Being made of a metal, the coil spring has advantages that the pressing force is less likely to be decreased by temporal change due to little settling and little effect of temperature and humidity. This enables provision of a vibration-type actuator that delivers stable friction torque for a long time.
In a conventional structure of a pressurization unit of the vibration-type actuator, end turns at both end surfaces of the coil spring of a pressurization spring are polished. This causes a problem that, since the thickness of an end turn portion at an end portion of the coil spring during assembly becomes smaller than a gap to be formed after assembly of a rotor and a gear, if the coil spring is bent, the end turn portion will enter and be caught in the gap between the rotor and the gear. Thus, friction contact surfaces are not uniformly pressed to cause uneven rotation. Furthermore, the surface pressure increases partially to increase abrasion of the friction contact surfaces. As illustrated in FIG. 5, the coil spring of the pressurization spring has an end portion of winding, and the pressing force of that portion becomes different from those of other portions of the end surface to cause pressure unevenness. When the number of turns in the end turn portion is one, the rigidity difference between a spring receiving member and a surface that is in contact with the spring receiving member becomes 10 times or more. This results in an uneven friction contact state to cause uneven abrasion and driving. Furthermore, when side surface portions other than the end turn portion of the coil spring interfere with other components, pressure unevenness and abnormal noise are produced.
The foregoing matter will be described below in more detail with reference to an example of a structure of a pressurization unit of a vibration-type actuator illustrated in FIG. 4. As illustrated in FIG. 4, a spring receiving member 114A of a gear 109 is connected to a coil spring 110 at an outer side of the coil spring 110. Between the gear 109 and a rotor 107 is formed a gap 118 of a certain size to allow the rotor 107 to move freely in the thrust direction with respect to the gear 109. Since the thickness of an end turn portion 110A at an end portion of the coil spring 110 is smaller than the gap 118 to be formed after assembly of the rotor 107 and the gear 109, if the coil spring 110 is bent during assembly, the end turn portion 110A will enter and be caught in the gap between the rotor 107 and the gear 109. The gap 118 between the gear 109 and the rotor 107 has a role to prevent a contact spring 108 from being pressed against a friction driving member 112 with excessive force during assembly.